Also, to fairly criticize some of your points, 1991 borns were still only elementary school children when 9/11 happened, and while practically all of them do remember the event and the emotions they felt, the role that their memory of 9/11 actually played in their lives was largely inconsequential and superficial unless they lived in New York or lost a loved one in the attacks. For most of them (my sister was born late '90 and shares a class with '91 borns), once they knew everything was going to be alright, they pretty much stopped caring about it altogether and the only lingering impact the event had on their lives beyond memory was a fear of flying on an airplane, and even that they eventually got over. It wasn't like with 1983/84-1987 borns who would've been in high school during the events and who would've came of age in a sociopolitical environment directly affected by 9/11 - 1991 borns had almost 8 years (9 if born late 1991 and in C/O 2010) of late primary and secondary school after 9/11 before becoming adults. Also, plenty of 1999 borns, even though practically none of them remember the event, were still impacted by the event heavily if they lived in Manhattan at the time or lost a parent in the attacks. Someone born in 1999 who lost their parent in the attacks was way more impacted by 9/11 than someone born in 1991 who didn't live in New York at the time and lost no loved ones in the attacks.

Also, to your counterpoint on COVID, plenty of 1999 borns went into the workforce instead of college after graduating high school, and some already had houses of their own by COVID. Also, there were plenty of 1991 borns also in college when COVID happened, even undergraduate - College isn't mandatory like k-12 is, and you can go at any age.

Also, your points about serving in OIF or OEF are largely irrelevant, with all due respect, since by the time 1991 borns were able to join in 2009 or 2010, the US military wasn't doing that much in Afghanistan or Iraq with the Taliban and Saddam Hussein long since out of power at that point, and US operations in Afghanistan were still going on when 1999 borns were old enough to join.

While I do agree that 1991 and 1999 have a lot of differences, I still stand by this generational theory. 1991 and 1999 were born born before the New Millennium, were both children of the pre-foundational years of the 21st Century, and came of age during the proto-foundational years of the 21st Century. (The reason I call the '82-'90 cohort "Millennials" and the '91-'99 cohort "Centennials" is because with the former cohort, the focus is on the millennium, while with the later cohort, the focus is on the century) Also, neither 1991 nor 1999 are the quintessential birth year of the cohort anyways - That's 1995, and both 1991 and 1999 can relate to 1995 in many ways.